Factoring out the div with hardcoded style generated by form_tag

Dear Rails developers,

I've been troubled by the fact that form_tag generates a <div> with a
hardcoded style attribute for my forms. I created a patch that pulls
out the div-generating code into a public method so that (1) plugins
can redefine the method, and (2) the method can be reused for wrapping
other hidden_field_tags.

The lighthouse ticket is below. I'm looking forward to your feedback.

Thank you,